Wide body aircraft, such as for example the AIRBUS A 310-300, are already known, which possess an adjustable horizontal stabilizer in which, in order to increase the radius of action, an additional fuel tank has been disposed. Generally, in such aircraft, the centre of gravity is located in front of the point of application of the lift (focus) and the stabilizer lift is in fact a downwardly directed negative lift. As a result, the lift of the wing unit must be increased by as much, which involves an additional drag. In order to improve the performances of the aircraft by reducing the drag, it is necessary to move the centre of gravity back as much as possible. In fact, the more the centre of gravity is moved back, the less the moment exerted by the stabilizer negative lift with respect to the centre of gravity and therefore the less the additional lift and drag.
In these aircraft, the additional fuel tank of the adjustable horizontal stabilizer is thus in connection with the principal fuel tank arranged in the wing unit and a system for transferring fuel, incorporating regulator, between said additional and principal tanks is provided. Consequently, the quantity of fuel contained in the additional tank may be regulated so that the centre of gravity of the aircraft is longitudinally as far back as possible, whilst remaining within a range in which the aircraft is manually pilotable. If, for any reason, the system ensuring regulation deviated, the centering of the aircraft might become If the centre of gravity is moved too much to the rear, the aircraft would become unstable, and, the aircraft would become difficult to pilot due to the excessive demands on the pilots.
Said regulator thus controls the transfer of the fuel from the principal front tank towards the additional rear tank, or vice versa, so as to reduce the static margin, by placing and maintaining the centre of gravity of the aircraft as far to the rear as possible, whilst keeping a margin vis-a-vis the point where the aircraft is no longer pilotable. In practice, it is desirable to maintain the centre of gravity near the focus, and sometimes between the focus and the point of manoeuvring, which lies to the rear of said focus and which is defined as the point where the elevator has an infinite efficiency on the load factor of the aircraft.